1. Technical Field
The present invention relates to an image reading device and an image reading method.
2. Related Art
JP-A-1994-178059 disclose an image reading device which reads a document as an image in a manner such that light is emitted from a light source unit having a light source to a document, and reflected light which is light reflected from the document or transmitted light which is light transmitted through the document is inputted in an image sensor. As a light source of the light source unit of the image reading device, a technique using a light emitting diode (LED) is known (for example U.S. Pat. No. 5,767,979).
Generally, an image reading device reads an image by repeatedly performing a series of processes, such as accumulating charges in light receiving elements of an image sensor while transporting the image sensor relative to a document, reading an amount of accumulated charges, and producing image data on the basis of the read amount of accumulated charges. When a resolution is determined, transporting speed of the image sensor and charge accumulating period are determined such that the series of processes can be performed without delay. In the case in which an intensity of light of the LED serving as a light source is strong, if the LED is lighted for the entire time of the charge accumulating period, the light receiving element may be saturated. For this reason, JP-A-2005-203924 discloses a technique in which the LED is lighted for only a predetermined time from a starting point of the charge accumulating period, and then the LED is put out after that.
However, the known technique does not mention the relative transporting speed of the image sensor to the document. Accordingly, if the relative transporting speed of the image sensor to the document changes in the middle of image processing, image quality of the read image is likely to be deteriorated. This problem does not occur only in the case of using the LED as a light source but also commonly arises when using different light sources, such as a fluorescent lamp.